Abstract

Some aspects of the noise generated internally by a turbojet
engine are considered analytically and experimentally. The emphasis
is placed on the interaction of pressure fluctuations and entropy fluctuations,
produced by the combustion process in the engine, with
gradients in the mean flow through the turbine blades or the exhaust
nozzle.

The one-dimensional interaction of pressure fluctuations and
entropy fluctuations with a subsonic nozzle is solved analytically. The
acoustic waves produced by each of three independent disturbances
are investigated. It is seen that results for a large number of physically
interesting nozzles may be presented in a concise manner.

Some of the second-order effects which result from the area
variations in a nozzle are investigated analytically. The interaction
of an entropy wave with a small area variation is investigated and the
two-dimensional duct modes, which propagate away from the nozzle,
are calculated.

An experiment is described in which one-dimensional acoustic
waves and entropy waves are made to interact with a subsonic nozzle.
The response of the nozzle to these disturbances is measured and
compared with the response as calculated by the analytical model.

The interaction of two-dimensional entropy waves with a subsonic
nozzle and with a supersonic nozzle is investigated experimentally.
The results are explained in terms of an analysis of the acoustic
waves and entropy waves produced by a region of arbitrary heat
addition in a duct with flow.